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More On the Physics of Air Movement

The principle of using canted blades revolving radially to move air is centuries old and followed as the reverse of the ubiquitous windmill. In other words instead of using moving air to create mechanical movement, energy applied to the blade arrangement could move air for cooling or ventilation purposes.

Fans for creating more comfortable environments for people were limited by available energy sources until the development of electricity and relatively cheap electric motors. To move more air, all that seemed to be required were faster, more powerful motors.

At some point however, engineers became so focused on using speed to increase fan displacement - the cubic metres of air per minute moved through the fan - that some important physics-based issues that impact on human comfort from fans were overlooked:

High velocity air movement - wind - is both unpleasant and disruptive.
Air speed beyond six to ten km/h offers little, if any, additional cooling benefit.
In very hot, low humidity conditions, very slow moving air actually cools best.
Small high-speed fans create a pressure differential that is essential for many applications, but where slow movement of free air is the objective, pressure differential is not important.
Displacement; the amount of air actually moved through the fan, is of no real significance. It's the down-stream effects that are important. A turbulent, high velocity air jet dissipates very quickly.
A large column of air "travels" farther than a small one. The friction between moving and stationary air occurs at the periphery of the moving column. The perimeter of a column varies directly with the diameter. And while the cross-sectional area varies with the square of the diameter, the large column has proportionately less periphery, and therefore, less "drag." The air column from a 900mm diameter fan has more than 6 times as much "friction interface" per M³ as a the air column from a 6M fan.
Because of this far-reaching effect, large low-speed fans, properly arranged, are capable of establishing continuously circulating air currents that transport vastly more air than smaller high-speed fans of equivalent displacement.
The power to drive a fan increases roughly with the cube of the average air speed through the fan.
A fan delivering air at 30 km/h requires about 64 times as much power as one the same size delivering air at 8 km/h!

This, combined with the "effectiveness" factors, means that when free air movement for cooling people is the objective, low-speed fans are enormously more energy efficient than high-speed fans.