Larry,

 

While I have been involved in several perchloric acid fume hood designs, this question has never come up before.  I have always strived to make the exhaust system as small as possible and never combine hoods, i.e. separate systems for each hood and always tried to locate just below the roof level when possible.  I did not find any specific reference but I am limited in that I am working remotely away from the office this week.  I did find the following in the ASHRAE Applications manual:

 

.Perchloric Acid. Standard hood with special integral work surfaces, coved corners, and non-organic lining materials. Perchloric acid is an extremely active oxidizing agent. Its vapors can form unstable deposits in the ductwork that present a potential explosion hazard. To alleviatethis hazard, the exhaust system must be equipped with an internal water washdown and drainage system, and the ductwork must be constructed of smooth, impervious, cleanable materials that are resistant to acid attack. The internal washdown system must completely flush the ductwork, exhaust fan, discharge stack, and fume hood inner surfaces. Ductwork should be kept as short as possible with minimum elbows. Perchloric acidperchloric acid is an extremely active oxidizing agent, organic materials should not be used in the exhaust system in places such as joints and gaskets. Ducts should be constructed of a stainless steel material, with a chromium and nickel content not less than that of 316 stainless steel, or of a suitable nonmetallic material. Joints should be welded and ground smooth. A perchloric acid exhaust system should only be used for work involving perchloric acid.

 

.The general air distribution and exhaust systems should be constructed of conventional materials following standarddesigns for the type of systems used. Exhaust systems serving hoods in which radioactive materials, volatile solvents, and strong oxidizing agents such as perchloric acid are used should be made of stainless steel. Washdown facilities and dedicated exhaust fans should be provided for hoods and ducts handling perchloric acid.

Hood use may dictate other duct materials. Hoods in which radioactive or infectious materials are to be used must be equipped withhigh-efficiency (HEPA) filters for the exhaust and have a procedure and equipment for safe removal and replacement of contaminatedfilters. Exhaust duct routing should be as short as possible with minimal horizontal offsets and, when possible, duct portions with contaminated air should bemaintained under negative pressure (e.g., locate fan on clean side of filter). This applies especially to perchloric acid hoods because of the extremely hazardous, explosive nature of this material.

 

The mention of limiting horizontal offsets is the closest reference I found for something like 45 degree runs. A 45 degree run might need additional spray nozzles to ensure coverage of the top of the ductwork in the 45 degree run.  Whereas, avertical run would seem to allow the washdown system to cover all of the ductwork internal surfaces easier.  Hope this information helps, feel free to call if you want to discuss more.

 

Stephen Waller PE, LEED AP	HDR Architecture, Inc. Sr. Vice President, Director of S&T Engineering
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