My Googlesearch revealed that many academic institutions have design standards thatrequire that the design of perchloric acid fume hoods conform with the standard ANSI/AIHI-2003.  The standard is available from Amazon at a costof $83.  Stanford University is among those requiring conformance to the standard.  With respect to the ventilation system, the Stanford design criteria include the following "performance-based" text (verbatim):"Ductwork for perchloric acid hoods and exhaust systems shall takethe shortest
and straightest path to the outside of the building and shall not be manifolded
with other exhaust systems. Horizontal runs shall be as short as possible, with
no sharp turns or bends. The duct work shall provide a positive drainage slope
back into the hood. Duct shall consist of sealed sections. Flexible connectors
shall not be used."

Based on the foregoing, good engineering judgement must be applied with respect to the potential negative impact of a specific 45-degree bend (is it in a vertical or horizontal/sloped run? are there multiple 45 degree bends? does the presence of the bends impede the downflow of water? etc.).   The significance of one or more 45-degree bend(s) must be considered in the context of the design as a whole. The standard is "performance-based."

David Haugen
Argonne National Laboratory - Retired

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From: "Larry McLouth" <ldmclouth**At_Symbol_Here**LBL.GOV>
To: DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU
Sent: Wednesday, December 14, 2011 10:14:27 AM
Subject: [DCHAS-L] Perchloric Acid Hood Duct Design Criteria

Hi All -

Is anyone aware of a prohibition against using 45 degree angle ductwork
runs (as opposed to vertical runs) for a perchloric acid fume hood?  
This pertains to water washdown systems.  This question was posed to me
and I thought I'd seek your collective wisdom.I'd appreciate it if you could point me to any references you may be
aware of regarding this subject.

Regards

Larry