Need advice on fan size for ventilation

[Cosmo]

I'm just about finished building my workshop at my house. The building is 12' x 12'. Inside it's about 11' x 11' or so. The back wall is going to be a bench where I can put up to three torches. The bench is going to protrude 36" from the wall.

For ventilation, I think the easiest thing is to build a hood all the way across, arc it to follow the roof line, and put the fan at the highest point (but still under the roof). I need to know what CFM fan I should be looking for, or if I should consider a different kind of hood, or whatever.

The fan will be a centrifugal blower type fan, which will exhaust out the back wall of the building.

I came up with this somewhat crude drawing to explain what I'm trying to do:

[Dale M.]

How much space from top of bench to lower edge of hood?

Dale

[Cosmo]

I'm not quite sure yet. I'm going to guess and say about 4 feet. It should be in that range.

[Dale M.]

At 4 feet X 11 Feet, you are talking 44 sqft "face opening". That by most rules it would require about 4,400cfm... Some big numbers....

Dale

[Cosmo]

Would the height of the torches from the bench matter? I generally work standing up, so my torch would be a foot or so above the bench by the time I get it where I need it.

I've never heard of the distance from the hood to the bench being discussed before, so this part of it is new to me.

[Dale M.]

The concept is "Face Opening" it is defined by the frontal area of space that torch is sitting in.... So if your face opening (the height of opening from bench top to lower edge of hood and from the left side to right side of opening) is "x" square feet, simple ventilation rule is that you have to have 80 to 120 cfm of air movement per "x" square feet ... I prefer to use 100sqft for my calculations...

So if dimension of face opening is 10 sqft (2ftx5ft=10 sqft x 100cfm= 1000sfm) then the "simple" airflow at face would be 1000cfm... You also have to take into account for static pressure in duct (turbulence and back pressure (resistance to air flow)) and factor you need to over come. So exhaust duct sizes become critical too.

Most of these calculation are consistant with Class "B" fume hood uses in industry that have same general hazards as we are subject to in lampworking.

More on ventilation can be found here....

http://www.artglassanswers.com/forum/viewtopic.php?t=153

Dale

[Cosmo]

Gotcha. Yeah, I knew I was going to be working with a large volume of air, so I eliminated any ducting just to make it simpler.

[Dale M.]

Another phrase to add to the confusion is "face velocity", its actually the cfm x sqft... It is same concept as face opening but just different words, a slight different what of stating things. Its the speed of the air moving through the "face opening"..

You may want to run your hood ideas by Vince over on ISGB forum and see what hey says....

Also Mike A. has done some writing on the subject...

Dale

[Cosmo]

Yeah, I read what Mike wrote, but all those numbers get confusing to me. My mind isn't what it used to be (which is bad since I'm only 33).

[Dale M.]

Just wait till you get to 62..... Mind, whats that?

Dale