I recently measured the breeze from our 25HP air compressor. 
The "intercooler" intake runs at 50 mph, with the exhaust running 25 mph - and a 10 deg F temp rise. 
1800 CFM - needless to say it makes racket!

The higher the speed, the higher the friction losses and noise. 
http://www.hartandcooley.com/flex/Air%20Duct%20Calculator.xls   
This recommends 900 ft/m MAX for residential - of course thats for heating/cooling not fume extraction.

http://www.elitesoft.com/web/newsroom/vent.htm
"These unique components not only require special consideration in calculating their pressure loss, they also greatly influence the design of the duct system. For example, a hood usually has slots through which particulate or gases are drawn through. For the hood to work properly, the connecting ductwork must allow sufficient velocity (typically 3,500-4,500 fpm) so that the particulate stays in suspension of the transporting air. "


>>> apptech@paradise.net.nz 24 Jan 08 01:41:22 >>>
> So, I'm putting in (finally) a new-to-me wave solder 
> machine.   It has a
> 4" duct attachment, and the manual indicates that some 
> sort of exhaust
> fan capable of 400CFM needs to be attached.


400 CFM into about 0.1 0.1 foot^2 = 4000 fpm at no 
compression or about 65 fps or 45 mph. Even with some 
compression that's "quite a gale".

That seems to be an immense amount of air for ventilation 
alone. Is there perhaps a factor of 10 or even 60 (cfh) 
involved?


            Russell




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