Ok, for a 24' ceiling . . . wow that's high.
Your best option would be to get some industrial fans. They are designed to me mounted on high ceilings, and move far more air than conventional fans, using less energy.
Here is some more information on industrial fans:
http://www.ceiling-fans-n-more.com/i...iling-fans.php
Otherwise, make sure you get very high quality ceiling fans. Casablancas with the XLP motor, Emersons with the K55, or American made fans. Here is more information on the American made fans:
http://www.ceiling-fans-n-more.com/a...iling-fans.php
As far as the ceiling height . . . 24' for a residential (non-industrial) fan is a tough one. Most companies will claim you want the fan 8-10' from the floor, but that would look ridiculous on your ceiling. Without doing any calculations I would consider an 8' downrod. But you can find more information on ceiling heigh considerations, as well as quality differences in fans and air movement, in the third and final link I am suggesting:
http://www.ceiling-fans-n-more.com/a...iling-fans.php
Hope this helps!
Dan