Oh goodness Lara - go back over the ventilation threads here on LE and over on AGF (
www.artglassforum.com) . I've written many thousands of words on ventilation over the past six to nine months or so. Basically, it boils down to this:
Use as large of ducting as possible (8" or better)
Use smooth sided galvanized metal ducting
DO NOT use flexible metal or plastic ducting
DO NOT reduce or increase the ducting size at any point in the duct run
Make the duct run as straight and level as possible
Try to avoid sharp bends, use 45 degree bends where possible
If possible, have the entire system 'numbered out' - measure the face opening of your hood, the total length of the duct run, and the number of bends and what angle - then shoot me an e-mail. I'll send back the CFM requirements for your hood, and the recommended diameter of the ducting for the installation you've got.
For fresh air, an 8" duct provides about 350 CFM of "free flow" fresh air. If you need more, you can increase the size of the duct, or you can power push fresh air in (which I personally don't recommend).