I have a 600w cool tube. I originally installed an 8 inch inline fan from HD. I noticed at the exhaust end there was hardly any movement. I could still touch the duct work surrounding the cool tube but it was indeed hot. Last night I was looking at my Stanley squirrel cage type fan and decided to work on a project today that was well worth the time and effort.
I constructed a box 13"x12"x12" made of wood that the fan would fit into. I cut a hole in the box exactly where the intake is on the fan when it fits inside. For the exhaust port of the fan I placed a 6" to 4" conversion duct with the six inch side facing the box. I mounted the box to the grow room wall with the intake hole on the box alligned with the intake hole I had already cut in the wall. I hooked everything up and sweet Jesus, what a difference. It is a pleasure to see ventilation hose moving with so much air blowing through it. Now when I feel the cooltube duct work it is cool to the touch and that was with the fan setting on low!
The Stanley fan only cost $45. A high output inline fan was about $130. After spending so much money on lighting, grow room, soil so on and so on, I didn't want to spend another $130. It was an easy project and worth the time!