I don't know what the face velocity listed for your specific hoods, but 
be
very careful of the placements of each of those hoods in relation to the
ventilation system's air currents and drafts.  Even the best of the 
hoods
will be negatively affected by air currents blowing into or across the 
faces
of the hoods.  Low flow hoods are at higher risk of non-capture than the
standard hoods.

If the hood is in a very quiet air current area, the specs for that hood 
may
be OK.  If even in the slightest doubt of the efficacy of the total 
intake
capture velocity at the hood face for whatever hood sash height is used, 
a
smoke test should always be used in addition to the average velocity
measured.

It takes only a very small draft of air blowing across the face to reach 
in
and negate one area of the open sash to reduce or negate the inflow 
velocity
allowing for contents to escape.  I am speaking from experience where 
many
hoods in our labs were badly placed and were negatively influenced by
drafting air.

Use a smoke test at the open face of the hood to determine the 
trajectory of
the air capture (or non-capture) across the opening of the sash to be 
safe.
Ken Smith
CIH, Retired

-----Original Message-----
From: DCHAS-L Discussion List [mailto:DCHAS-L**At_Symbol_Here**list.uvm.edu] On Behalf Of
Barbara Mowery
Sent: Monday, October 05, 2009 1:42 PM
To: DCHAS-L**At_Symbol_Here**LIST.UVM.EDU
Subject: [DCHAS-L] Low-flow hoods

Dear Colleagues,

I am doing the annual update of our Chemical Hygiene Plan, which 
specifies
face velocity rates for fume hoods. However, our newly remodeled Organic 
Lab
has energy-efficient, low-flow hoods. What is an appropriate value of 
face
velocity to specify for such hoods?

Barbara Mowery 
General Chemistry Laboratory Coordinator 
Physical Sciences Department 
York College of Pennsylvania 
441 Country Club Road 
York PA 17403-3651
717-815-6480
Fax 717-849-1653

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