Truth about CFM/Watt

6-1-18 I was asked about using attic fans (also called gable fans) as a less expensive way to cool an industrial space rather than buying the more costly rooftop mounted whole house fan. It was suggested that buying the QuietCool attic fan AFG-PRO-3.0X and adding duct to it would be more cost effective.  This attic fan has specs on the web site that I reprint here.

The spec sheet top line says 250 Watts  and 3013 CFM
Variable 10-speed ECM motor Automatically adjustable fan speed 

But lower on the spec sheet it says 
Up to 2830 CFM on high at 145 Watts Down to 963 CFM at 10 Watts

I did some research and discovered that the published specs for that gable fan appear to be in error. You will find that there is another fan, the PRO-3.0X that is reated at 3013 CFM at 250 watts. The SMT is rated at 2830 CFM at 96 watts. 

I did some research into the possibility that one could get 2830 CFM with just 96 watts. Here is a chart showing 3 top sources for fans. First, note that gable fans are designed for free air flow, not ducted flow, which means that the air flow is not restricted by having to move through a tube with a very rough internal surface. The duct causes what is known as static pressure which slows the air down and makes the motor work harder for the same air flow as in free air. 

A perfect example of this is the Thorwaldson ES-6400 compared to the ER-8000. They both use exactly the same motor and blade, yet the 8000, since it has no duct, is able to move more air at the same input power. 26.7 percent higher to be precise (1685 more CFM out of 6320)

Next, I documented each ES series fan from QuietCool and each free air fan from Soler Palau to get another look at the industry capability. The results are here. You can click on the picture for a larger view. You see that the left columns are all free air flow. The right columns are ducted. You can see that the gable fan in question, number 2, sticks out as expected because the data they used must have been in error.