Local Exhaust for Welding

[Pulpboy]

I have to design / upgrade a welding fume removal system. What I don't understand is how the capture velocity works. If I want 200fpm capture velocity through a hood of Ce 0.98 (bell mouth) how do I calculate how many cfm and pressure difference I need. I have the Industrial Ventilation book by the American Conference of Governmental Industrial Hygienists but I must be missing something in my calculation. Can someone walk me through a calculation.

Particulars: 200fpm capture velocity
12" Diam (opening of bell mouth)
Ce ~ .98
Ducting some arbitrary effective length.
Duct velocity 2000 fpm

Thanks for any help.

 

[Koske]

You are suppose to get capture velocity when you divide hood area with air flow through hood. Actually you are suppose to determine how much air you want to exhaust. Then you have to divide exhaust air flow with the hood capture velocity and that will give you the hood required area.

 

[russmech]

Whatever you want to capture (smoke) should experience the capture velocity at the point you want to capture it. And the velocity of air from a source decreases as the square of the distance from the exhaust source (end of the duct). That's why, in exhaust applications, distance from source to capture is everything and is why you want a small hood close to your source and not a large hood a long ways away. Assume a distance from source (welding) to the end of the duct and calculate how much air you have to move in order to achieve that using Figure 3-7 of Industrial Ventialtion (showing the velocity contours vs distance from end of duct. Do yourself a favor and buy an articulating welding hood that can be moved around as the welder requires. Search the net for "exhaust snorkels"