Because there are so many ways (some quite expensive) to measure and so many factors that affect a fan’s CFM rating, we recommend using thrust to compare fan performance, not just CFM. We like thrust as a measure of fan performance because it is inexpensive, there are fewer variables to affect results, it is based on a LAW of physics, and anybody can measure it consistently between multiple fans to determine relative performance accurately.
Thanks to Newton’s 3rd Law of Motion (for every action there is an equal and opposite reaction), it follows that the more air a fan pushes, the harder the fan will also be pushed backwards. Fans must be engineered in such a way as to handle that thrust without walking, rotating, or falling over!
At right is a flyer describing our thrustometer or thrust-measurement apparatus. A simplified version can be done by setting a fan on top of a piece of plywood and the plywood on top of a couple of pipes or broomsticks (rollers). Attach a fish scale to the front of the guard and to something solid in front of the fan so as the fan pushes backwards, it pulls on the scale.
Click here to learn of another simple test that anyone can do to measure fan performance under different conditions.
Note: To approximate the volume of air being pushed, multiply pounds of thrust by a little over a thousand cubic feet/pound.
As a general rule of thumb, you should have 11–15 pounds of thrust for residential fires, 15–18 pounds for commercial fires, and 18+ for industrial fires.
Dad and James needed to move a couple docks across Hauser Lake, about a mile. They first used a small boat supplemented by our shop fan, a propane powered 24GX200 VENTRY Fan (LP fans are no longer available for purchase). At the end of the journey, they had to maneuver the docks down a long, narrow canal to the beach, so they just let the fan do it all. It worked remarkably well!
In these photos (click them for larger view) you can see the wake of the docks moving through the water. Using this VENTRY Fan was equivalent to standing in the water and pushing against the docks with 20 pounds of force. Thrust is a great thing, as long as your fan is designed to handle it without tipping over, as VENTRY Fans are.