From: Monona Rossol <actsnyc**At_Symbol_Here**>
Subject: Re: [DCHAS-L] Fume hoods purge buttons
Date: Tue, 13 Aug 2013 18:27:22 -0400
Reply-To: DCHAS-L <DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU>
Message-ID: 8D0668B69E426F1-142C-25176**At_Symbol_Here**
In-Reply-To <5564F9EDC11C09468EE5DAF02B5CB30F4A8216D2**At_Symbol_Here**>

In fact, that's exactly what I'll be thinking about.  Our displacement systems usually are set up to provide supply at just about head level on one side of the room with exhaust on the opposite side of the room via a countertop slot hood and a large floor to reach level perforated plenum exhaust behind a large drying rack.  So we don't have to consider much of the ceiling area in our calculations.  The displacement is mostly in the breathing zone and near the floor.  Since painters use somewhere around an ounce or two of solvent on canvases and tabards in the open, I have done the calculations for this many times.  We recommend a particular solvent because the molecular weight of the solvent, evaporation rate, TLV,  etc., are all part of the calculations.
So If this is primarily the only solvent in the room (TLV 100 ppm)  with perhaps a little turpentine thrown in (TLV 20 ppm), I should be able to figure a rate for a spill of a gallon of one and a pint of the other which would be worst case scenario.
I will admit we have a much easier issue here than you people who must plan for gosh only knows what chemical.  But even without knowing all of the parameters, I don't see a lot of down side to getting the air moving as fast as possible through the same duct work, explosion-proof fan, and stack on the roof that is used for regular exhaust.. 
Monona Rossol, M.S., M.F.A., Industrial Hygienist
President:  Arts, Crafts & Theater Safety, Inc.
Safety Officer: Local USA829, IATSE
181 Thompson St., #23
New York, NY 10012     212-777-0062

-----Original Message-----
From: Ralph B. Stuart <rstuart**At_Symbol_Here**CORNELL.EDU>
Sent: Tue, Aug 13, 2013 1:53 pm
Subject: Re: [DCHAS-L] Fume hoods purge buttons

> And I can see how a purge system that would keep the atmosphere in a lab somewhat under control in an emergency could be useful.  A purge system might preclude a spill getting out of hand to the point that SCBA would be necessary.  Or it could prevent a solvent spill vapor concentration from ever reaching the LEL.  
I guess my question is: What is the ventilation rate associated with "purge" mode? I can understand how some assumptions about this could be made in a specific setting such as theater stage with a limited set of emission scenarios to consider. However, in a generic laboratory with multiple potential pollutants located in a variety of places, I think that a design engineer would have a harder time specifying what higher flow rate is appropriate. It would depend on the hazard being protected against (e.g.. flammability or toxicity) as well as the geometry of the room. Of course, we have similar concerns in laboratories under normal operating conditions, as the variability in laboratory ventilation effectiveness applies to that situation as well-
- Ralph
Ralph Stuart CIH
Chemical Hygiene Officer
Department of Environmental Health and Safety
Cornell University

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