Steve:

Based on the size of the system that you describe, the recommended flowrate of 4,000 cfm sounds reasonable.  Check out this link.  These guys manufacture dust collectors for these types of applications.

http://www.farrapc.com/

Good Luck.

Tom

When designing an exhaust system, both volume of air (CFM) and velocity (FPM) have to be considered as well as friction losses through the ducts. Neglecting friction losses,from what you have described, a 4000 CFM fan sounds about right as rough calculations show that you would have a capture velocity at the filters of around 650 – 750 FPM,which is good.   The only thing that I have reservations about is the hood outlet dia. The outlet being around 9” (225mm) dia, would give you a stack velocity of around 9000 FPM ! Ducts tend to get noisy when you exceed 2500 – 3000 FPM. 9000 FPM would start to sound like a jet engine.  For a stack velocity of 2500 FPM your hood outlet dia should be about 17 – 18”.  To arrive at an outlet dia to provide a desired velocity, use this formula:

Fan volume in CFM/desired duct velocity in FPM = Cross sectional area of duct in sq. ft.

For a round duct,

(Sq root (4000/2500*144)/pi)*2

For a square duct,

Sq root (4000/2500*144)
   
All this being said, for efficient fume extraction, the design and location of the hood is important. Maybe this is why you say that the shop where you saw this set up, wasn’t getting good extraction.

Haggis  

You will spend more than 20 large, getting it right.

Pay the man.  Get the right stuff.

Else, find a tome on industrial toxicology, and look up 'metal fume fever' and related subjects.

Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

I am an industrial hygienist -- have seen laser cutters used for cuttin all kinds of metals including stainless (es).

One of the things to look out for is chrome in stainless which varies with the SS type.

IF in USA EPA will have some concern over chrome and of course the dust.  Then there is the politics with the neighbors ie "theyre killin us with chrome". I do not think just dumping the exhaust to ambient outside air is the proper answer.  Basically you made the mess in your shop clean it up in your shop.

OSHA has regulatins for chrome also.  I predict you should not have too much problem with these the smoke is going to be the major concern and issue.

I wonder if water spray and or electric precipitator would be better for you - eliminates cost of filters WITH other trade offs and expenses of course - ain't no free lunch - but that is cost of doing business.

Dan Bentler

I was the Containment specialist for Camfil UK and currently supply the UK MOD with Chromate protection / repairs.
If you have no joy, post here and I will give you some other contacts or see if I can help you myself
.
Be careful to ensure you use an industry specialist as dealing with Dusts, gases and Chromates will come under ATEX in Europe. Too many times installations do not meet legal requirements.

You will also need to consider Duty of care act very carefuly and a full risk assesment must be carried out to identify potential hazards

You will also require minimum H13 HEPA filtration for Chromates. 99.97% @ 0.3 microns EN1822 or DOP testing with certificates - circa 2000/4000M3/hr

If you are allowing Chromates to contaminate your factory space you may also need a full cleandown - In particular Chromium VI
Regular testing of all personel must be undertaken for Chromates.

Chromates, in particular VI are class 1 carcnogens and are present in many metals, paint, primers etc etc - hence wearing full breathing equipment.
In addition Chromates are also absobed in the tear ducts.

I believe the USA are reducing the 8 hour allowable limit from 5mg to 1mg from Jan 06



Hope this helps