There is an easy way to get to the answers to both questions, i.e. the 
generation rate and the room concentrations. The AIHA provides mathematical 
exposure modeling software called IHMOD. It is set up to calculate the 
evaporation rate and along with that offers a wide range of exposure models  
including well mixed box models, two zone models,  eddy diffusion etc. There are 
models that use constant generation rate or exponentially decreasing generation 
rate (where the size of the evaporating source is decreasing as evaporation 
occurs). The software is free and is an Excel application. Just search on "AIHA 
IHMOD download" and you will see the AIHA download page. The models have 
"sliders" that allow you to see the effect of decreasing or increasing any of 
the input parameters. So to see the impact of less of more ventilation or a 
smaller or larger spill is very easy and the results are shown graphically. It 
runs on most any version of Excel.



Jim



James H Stewart, Ph.D., CIH, CSP

Director, Building Science and Forensics

Environmental Health & Engineering, Inc.

117 Fourth Ave

Needham, MA  02494

800-825-5343

________________________________
From: DCHAS-L Discussion List [dchas-l**At_Symbol_Here**MED.CORNELL.EDU] on behalf of Monona 
Rossol [actsnyc**At_Symbol_Here**CS.COM]
Sent: Tuesday, August 13, 2013 6:27 PM
To: DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU
Subject: Re: [DCHAS-L] Fume hoods purge buttons

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
actsnyc**At_Symbol_Here**cs.com<mailto:actsnyc**At_Symbol_Here**cs.com>   www.artscraftstheatersafety.org<http://www.artscraftstheatersafety..org>


-----Original Message-----
From: Ralph B. Stuart <rstuart**At_Symbol_Here**CORNELL.EDU>
To: DCHAS-L <DCHAS-L**At_Symbol_Here**MED.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

rstuart**At_Symbol_Here**cornell.edu<mailto:rstuart**At_Symbol_Here**cornell.edu>