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capture velocity

capture velocity

capture velocity

Does anyone know much about capture velocity?
Has anyone out there done any experimental work to determine what the capture velocities might be for various contaminants and various environmental conditions?

I have read what the Industrial Ventilation Manual says about this. I just want to know more.

Thanks for your help.

RE: capture velocity

I did several weeks of testing for a DoD tri-service metal working training facility, welding brazing, soldering, gouging, etc. Getting air arc gouging to come within OSHA Z and ACGIH limits was a gnarly job; it shoots copper around like hot butter, and Cu limits are low.

I would recommend looking at ANSI Z49.1 for more info. The COE had me deliver a paper on this in 1995-I don't have a copy anymore, but they might hve it on their web site.

Capture velocity means a lot more when you have capture distance included. OSHA and ACHIH do not cover that. I received a field interpretation from OSHA approving usage of 9" capture distance; after that, air arc gouging was the only problem. Had to do repetitive mock-ups with a millwright until the enclosed LEV would meet standards.

RE: capture velocity

Thanks for your helpful response.
I am curious. Were you ever able to control the contaminants from air arc gouging? If so how did you do it? Was the hood easy to reposition as the operator moved around? What size were the parts being worked on? I suppose they were fairly small if it was a training facility.

Thanks again.

RE: capture velocity

All contaminats for air arc were satisfied within action levels. For the air arc gouging, I believe we ended up with near full enclosure (no ADA compliance), push-pull ventilation, and back exhuast protected by refractory brick. Procedural restriction was also put in place that any and all arc gouges must be placed or maintained with forced air direction going to refractory bricks on the back exhaust openings (we used refractory brick to protect the steel). The ACGIH was also used for placing the refractory for slots to reach 100 fps at 9" from exhuast slot. First two iterations using 6" capture distance failed; going to 9" did cause problems for the bag-in/bag-out, ventilation system capacity, and ground fault analysis, basically all the mechanical, electrical, and air permit. Most of that came out when we could reduce exhaust to regular welding stations and soldering/brazing. Once the IH portion was satisfied, the traditional MEP began. The containment itself was not positionale, as it weighed several hundred pounds. Th refractory and slot size was adjustable.

The design was trial-and-error, prototyping variations from the ACGIH until we could demonstrate 100% passing on personals at the action level. Copper was the kicker on this one, and we used it as the baseline for passing all other analytes. Once the action level was met, no other analytes came near to STEL or action level.

Air arc gouging is a very messy business, and after training, the idea is the field work will be done outside. Up until that time, it had never been done in doors while meeting OSHA/ACGIH requirements. I beleive that the DOL, AIHA, and CERL also had papers presented and available off the web. I though about using computer based modeling at first, as alot of research was available out of Japan at the time; however, none of the Japanese (Kobayashi, I beleive) computer models passed in real life.

It wasn't really as much engineering as trail-and-error on an accelerated basis.

RE: capture velocity

100 fps or 100 fpm for capture velocity at 9" from slots? I assume you meant 100 fpm.

I may have an application where I need to control air arc gouging indoors. The problem is that the part is quite large so the worker is constantly moving around it. Local exhaust ventilation is probably not possible so general ventilation may be the way to go.

Thanks again for you help. Your work on that project sounds interesting.  

RE: capture velocity

You should search for articles which discuss "saltation velocity" and "choking velocity." These terms represent the horizontal and vertical velocity required to keep particles entrained in an airstream.  Obviously capture velocity at points away from a hood will be low compared to saltation and choking velocities.  You would have a very large volumetric flowrate if you were designing for the transport of larger particles outside of the face of a hood.

RE: capture velocity

Sorry, I meant fpm.

Wherever you're aiming the slag, that's where you will get the greatest exposure potential; the air wand pretty much overcame any normal dispersion, which was why directional control was possible. As it was a training facility, the enclosure, back exhaust and push exhaust were aligned in slag direction. Multiple position, and especially vertical, were not considered. Trying to move air away from gouging direction did not work well on any prototype. Containment (enclosed by steel) were used inside of general exhaust.

Handling the slag was also an issue. You are talking about shooting plasma in any direction. It's a very serious safety problem, and the slag ate up most surfaces (or rather deposited on them heavily). Best I could think of is refractory, which might not be doable at multiple positions.

It may have been 15 years ago, but at that time it was held to be undoable even for training purposes. The first two EOR's paid liability, so you are not going into a historically easy area. I hope you have a good CIH. Good luck going to production.

RE: capture velocity

dvd: Thanks for the advice. I will read up on that. Normally I would design a hood to achieve the desired capture velocity at the source. Once the contaminant is inside the dust control system, then I would need to maintain a certain carrying velocity inside the duct so that particulate doesn't collect inside it.

mauricestoker: I think I would opt for a general ventilation system. I don't think a local exhaust hood could work for this gouging application. I would bring in fresh air at about 4 feet off the floor facing the tradespeople and exhaust up high, diluting the contaminants to acceptable levels in the breathing zone.

RE: capture velocity

When I worked in the restaurant equipment business we manufactured hoods. The flow rates were covered under nfpa96 (long time ago), but for restaurant 'grease' 100cfm per sqft with a 1000fpm in the ductwork.
I also did some grinding dust collection equipment for use in house and found with a large shop vac we were able to get 1000fpm at 8" from the inlet (worked for nuisance dusts generated by grinding equipment- heavy particulate still collected on the floor...)

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