Calculating typical floor fan efficiency or dP? (Spec sheet too good to be true?)

Question

I am looking to purchase a fan with high velocity. I am trying to make solid data driven decision based on specs. I found that most fan's of the 20" diameter variety are around 3000 CFM; however found one that is stated at 6000 CFM (2.83 m^3/s) with 162 watt and 1450 RPM. Link to fan: https://www.homedepot.com/p/Commercial-Electric-20-in-3-Speed-High-Velocity-Floor-Fan-SFC1-500B/302845111

I am using equation efficiency = dP*flow rate/Power and using 50 Pa dP (not sure what is proper value there, but it is giving me reasonable efficiency ratings so I normalized to that) I am coming to an efficiency of 87% for this fan.

It seems like the rest of marketplace is more like 30 to 50% efficiency (again using the assumed dP of 50 Pa).

Can someone help me understand if this is possible efficiency for a floor fan or too good to be true? Am I using correct equations and dP estimate. This is not my area of expertise by any stretch, but from what I have found the efficiency seems implausible.

Many thanks!

Answer 1

Your confusion seems to be in assigning a dP at all. On the back side of the fan, the air is at 1 atmosphere, and on the front side, the air is at 1 atmosphere. There is no static pressure gain at 6,000 CFM. Instead, you would consider the velocity pressure. This is more challenging:

1. First, take the velocity at each point on the outlet of the fan. These measurement points should be as many as possible for the highest accuracy.
2. Square the velocity, and multiply by the area this point covers.
3. Add all of these together, and divide by the total area.
4. Multiply by the density and divide by 2.
5. Convert to Pascals.
6. Use this to define the efficiency.

As someone who has experienced these fans, a significant fraction of the air at high speeds winds up heading out radially or even slightly backwards. Thus, a different manufacturer may have wound up running such a measurement for CFM and including this additional airflow, whereas the others disregarded it, knowing it was useless vortexing. As such, it may simply be marketing.