1. First big issue is that duct work should never have right angle entries into a main (should be 30 degree entries), should never make any rapid changes of direction or increase or decrease in diameter rapidly. And the diameter of a duct is never a standard dimension, it is determined by the centerline velocity that you need to achieve for the particulate.
What I see in chemistry labs is a lot of bad practice in terms of changes in direction and right angle entries each one of which can lead to air turbulence in the duct and condensation or deposition of particulate. And I see all the ducts leading from the off-the-shelf hoods are the same diameter indicating they assume all of the ducts are transporting the same contaminants with the same particle size and characteristics. I smell the work of HVAC engineers here rather than industrial ventilation engineers. And I must say, I have a problem with using ASHRAE standards for fume hoods and ventilation systems in general.
2. The second problem is the centerline velocity of the ducts in the lab. That velocity has to be fast enough to keep what ever you are transporting moving rapidly enough not to deposit. Examples of those speeds are in table 5-1 in the ACGIH Manual of Recommended Practice for Industrial Ventilation.
Gases, vapors and smoke 1000 to 2000 feet/minute
fumes, metal smokes 2000 to 2500
fine light dusts 2500 to 3000
dry dusts and powders 3000 to 3500
grinding dusts 3500 to 4000 and etc up to 5000 cfm for heavy and sticky stuff.
Notice that there is nothing said about oil mist. This is because oil mists typically can be fume sized (0..1 to 5 microns) or much larger depending on how they are generated (I am very familiar with these due to the oil mist theatrical smoke/haze generators that can do this hot or cold and in any size needed.)
In addition, all oil mists are condensation and deposition risks. I usually recommend a centerline velocity of at least 3000 f/m if oil mist is involved.
In other words, if you are using the fume hoods for just any and all contaminants you generate, you really need to have the system designed to efficiently transport the largest of the particulates and the particulates with the worst transport issues.
So to tell you the truth, I would be surprised if you were NOT getting deposition of oil mist in ducts in the labs I've seen. And people who don't think they have this problem are just getting deposition in locations that don't drip out where you see it.
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
From: Richard Van Doren <RVandoren**At_Symbol_Here**METROSOLUTIONSUSA.COM>
To: DCHAS-L <DCHAS-L**At_Symbol_Here**PRINCETON.EDU>
Sent: Thu, Oct 12, 2017 5:24 pm
Subject: Re: [DCHAS-L] Fume Hood Ducting
OMG. We also experience the same thing. We are not sure what it's from. Our hood install has been only a year old and we have been using it for metals digestions
using EPA 3050B. I would like to know what is causing this. I have yet to take it apart to see what the root cause of this is.
Anyone have an answer?
Richard Van Doren
Quality Assurance Manager
423 Pacific Ave.., Ste. 101
Bremerton, WA 98337
Has anyone experienced accumulation of oil (perhaps mineral oil from a heated oil bath?) in fume hood duct work? Is this reasonable or even possible? Could a sufficient amount remain in the system for it to condense out in the ducts when
the exhaust fans were turned off for an extended period?
Murray State University Department of Chemistry
For more information about the DCHAS-L e-mail list, contact the Divisional secretary at secretary**At_Symbol_Here**dchas.org
Follow us on Twitter **At_Symbol_Here**acsdchas