those fans aren't very powerfull at all. if you have more than a few feet of duct they barely push the air through it at all. the centrifugal fans are 1000times better/stronger/more expensive. if you are using cfls that fan might due but if you have hid you should look into fans like these. you can get adapters for them as well.
these:http://www.plantlightinghydroponics....roducts_id=351
or even these are great, i have the 465 and it can cool a 1kw hps cooltube keeping temps in the upper 70's
http://www.plantlightinghydroponics....p?cPath=76_415