Date: Mon, 26 Dec 2011 13:58:44 -0500
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From: ACTSNYC**At_Symbol_Here**CS.COM
Subject: Re: Perchloric information
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I'm really conflicted about commenting on this question, because it's likely to provoke controversy that I may not have time to answer fully.  I have a lot of prep work for a building ventilation retrofit consult in Dubai and Abu Dhabi in early January.  Also, I rarely work on chemistry labs.  But I've specified my share of chemistry fume hoods and many other types of local exhaust system in art and theater shops and studios.

I'm a long time member of both ASHRAE and ACGIH.  It is clear that ASHRAE's primary expertise is defined in their name: heating, refrigeration and air-conditioning.  It is their standards I look to when designing the offices and public spaces in builldings.

But it is the ACGIH's Industrial Ventilation: A Manual of Recommended Practice that I use for dilution ventilation and local exhaust systems.  And since I have a particular interest in ventilation standards, I can tell you that ASHRAE 62-1981 to1999 referred engineers specifically to the ACGIH Manual for industrial systems.  But since then, ASHRAE has expanded their scope to include more and more industrial systems.  The reference to the ACGIH manual in ASHRAE-62 is now relegated to a foot note in a table. 

I'm going to step on toes here, but having consulted on over 40 buildings, I can say that the ASHRAE should not be relied on for industrial applications. And you scare me when you say that you "have always strived to make theexhaust system as small as possible and never combine hoods, i.e. separatesystems for each hood and always tried to locate just below the roof levelwhen possible."

While I'm all for as many dedicated separate systems as possible to avoid balancing issues, I'm not sure what you mean by "as small as possible" and "just below the roof level."  I would refer you to the ACGIH manual's chapter 6 on Design Issues the section on exhaust stacks.  The diameterof the exhaust stack is dictated by the centerline velocity that is neededfor the particular application--not much leeway there.  And the stackneeds to high enough to get into moving air space above the building and never rain capped--e.g., they can be off set or you can spec the fan to exhaust against the resistence of a stack that narrows to boost the exhaust plume into moving air space.  This second option usually is not worth it for most small industrial applications. 

The stack height will vary depending on the size and configuration of the roof and the building's surroundings.  One project I worked on needed higher stacks because the building was lower than buildings on either side of it.  It took a model smoke study to determine just how high they hadto be.

There are many designs to integrate stacks into the architecture so they will not be ofensive to the sensibilities of architects, but they've got to be high enough not to re-entrain.


In a message dated 12/26/2011 10:12:23 AM Eastern Standard Time, Stephen.Waller@HDRINC.COM writes:

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:

ASHRAE Applications CHAPTER 16.Perchloric Acid. Standard hood with special integral work surfaces, coved corners, and non-organiclining 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 alleviate this 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 areresistant 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 acid exhaust systems with longer duct runs may need a zoned washdown system to avoid water flow rates in excess of the capacity to drain water from the hood. Because perchloric 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.

ASHRAE Applications CHAPTER 8. The general air distribution and exhaust systems should be constructed of conventional materials following standard designs for the type of systems used. Exhaust systems serving hoods in which radioactive materials, volatile solvents, and strong oxidizingagents 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 with high-efficiency (HEPA) filters for the exhaust and have a procedure and equipment for safe removal and replacement of contaminated filters. Exhaust duct routing should be as short as possible with minimal horizontal offsets and, when possible, duct portions with contaminated air should be maintained 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 45degree run.  Whereas, a vertical run would seem to allow the washdownsystem 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

HDR Architecture, Inc.
Sr. Vice President, Director of S&T Engineering
7200 Wisconsin Ave, Suite 501 | Bethesda, MD 20814
301.771.7041 | c: 609.577.1867 |

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