Xaliuz, Obi-Wan is correct, for the least resistance (or loss of efficiency), straight line connections are the best. If you must turn or combine flows, the more gradual of a turn will offer the least resistance to the flow. Also the duct work (tees, elbows, and lines) need to be sized properly to lessen the resistance. An eight inch diameter duct will carry almost twice as much as a six inch diameter duct (pi X radius squared, 8"=50.265" squared and 6"=28.274" squared)There are adjustable sheet metal elbows, usually available at HVAC departments at hardware stores. They go from straight to 90 degrees by turning the sections (called gores in the sheet metal business) and come in sizes from 6" to 8' diameter.
Good luck with Your grow.