Fans and blowers are rated by CFM delivered with NO BACK-PRESSURE. Back-pressure is the force required to push air down a duct or through any other constriction. Louvers on light reflectors, air filters, anything that hinders the free flow of air caused back-pressure. It is not unusual that a typical blower connected to a simple flex duct pumps only 1/2 to 1/3 the air that it is rated for. Fans connected to ducting are even worse.
Always remember that to pull air out of a space with a fan or blower, you must provide a way for air to enter the space as well. Try this: open and close the door to a vented space and see if you hear the fan or blower change pitch. If so, you may not be providing enough inlet air. Air inlets are usually large and have filters over them to prevent bugs and dust from entering.
Seem like a lot of numbers? Remember: numbers are good. Numbers are natural. The world is full of numbers! Just remember this, numbers are like dogs: just don't show fear and you'll be OK! Here are a few application tips for now. More detailed information will require some quality time with my calculator and the stereo volume either way up or way down.
Fans
Use fans to freely move air only. Example: a fan mounted in a hole cut into a door or wall with large spaces on each side.
Do not use a fan to force air down a duct, they are poor at providing pressure.
If a filter is required, cut down a furnace filter. They are designed to minimize back-pressure.
With no filter or other hindrance to air flow, select a fan that is rated for the CFM needed.
Provide an air inlet into the space that is at least three times the size of the fan.
When using a filter, grill or other hindrance to air flow with a fan, select a fan with a CFM rating 50% to 100% higher. This is to account for back-pressure losses.
Blowers
Use the minimum of flex ducting. Use rigid aluminum ducting for straight runs and 90 degree turns wherever possible.
Remove any louvers that may exist on the HID reflector for maximum blower efficiency.
Provide an air inlet into the space that is at least eight times the area of the blower exit.
With any ducting attached, select a blower that is at least double the needed CFM to account for back-pressure losses.
St0ney
Hope this helps someone