Most of the time it can be accomplished by changing the pulleys. I'm not aware of any NFPA 96 approved direct drive fans, or at least I havn't come accross any that I can remember!
There are test instruments that will measure air movement. Again Grainger has them. Some of the building codes require tests to make sure the air movement is adaquate. But not in Eastern Oregon. I have a small job coming up that the inspector said it was OK to open a window for return air. He must use a different code book than mine.
Where/who has the formula to calulate pulley size and the effect on air movement?
Douglas Hicks
General Fire Equipment Co of Eastern Oregon, Inc