If using a cool tube theres no reason to exit out the back, and it might be too wide to get a couple of 90's in there - Ideally you would want the intake to come in and exhaust out the sides of the cabinet - a straight shot through the tube with a fan on either end - then a couple of PC fans might work, but not if there are any turns..

I don't think the PC fans are gonna cut it for you - plus they will be loud. If you want a powerful, virtually silent fan, check out the Panasonic Whisperline models. The only thing I hear is the air moving in the duct, the fan makes hardly any noise. A couple of 120mm fans will be much louder.
Tokudai Reviewed by Tokudai on . 250w hps in a cab, how much fan noise? ok, so cab will be about 3'wide, 6'tall, 2'deep basically, i'd much rather use my hps for my flowerin cab than cfl's, BUT i need to make the cab quiet, SILENT would be best. so i'm thinking, a cooltube, exhausting out the back with a pc fan (92mm or 120mm), on a variable-voltage wallwart. plus another 120mm fan for regular exhaust at the top of the cap, with just passive intake holes all along the bottom back here's my question: i really don't know how much heat the hps will put Rating: 5